Title: Temperature Effects on Density

Principle(s) Investigated: How density changes when adding or removing heat from the substance

Standards :

Grade Eight

Density and Buoyancy

1. All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept:
   1. Students know density is mass per unit volume.

Materials:

• 1 Hot pot
• Ice
• Dye – 1 Blue and 1 Red
• Pipette
• 1000ml beaker with 600 ml room temp water
• Styrofoam cup to fill 1/3 with boiling water and 15 drops red color)
• 60 ml beaker to create 20ml water/20ml ice and 15 drops blue color
• Towel

Procedure:

1. Fill one dropper with blue cold water. Poke the end of the dropper about halfway into the colorless room-temperature water.

2. While observing from the side, very gently squeeze the dropper so that the cold water slowly flows into the room-temperature water.

3. Fill another dropper with red hot water. Poke the end of the dropper about halfway into the room-temperature water.

4. While observing from the side, very gently squeeze the dropper so that the hot water slowly flows into the room-temperature water.

Student prior knowledge: The definition of density

Explanation: This lab gives students a visual understanding of what happens to the density of a fluid when you add heat or remove heat. By boiling water and adding red food coloring, the students can clearly see that when added to water at room temperature, the red, hot water floats to the top. The opposite is true when adding the blue, cold water.Since the students conduct the experiment themselves, this is a richer environment because the students are engaged and actively participate in this lab.

Since hot water is less dense due to volume expansion, it floats on top of the room temperature water, which has a density of 1g/ml. For the ice cold water, its density is larger and sinks to the bottom of the room temperature water. Eventually, equilibrium is reached and the red/blue water mixes after creating a small convection current and the end result is purple water.

Density of a fluid is the mass/volume.

Questions & Answers:

· What did you learn about temperature effects on density?

When heating a liquid, its volume expands and it density decreases due to the mass per unit volume relationship.

· Will the different temperatures of water mix or form layers?

At first they will form layers and then eventually will mix after creating a convection current.

· What real life example causes you to believe this?

Unheated swimming pools in the summer have warm water on the top but as you go deeper it feels colder.

Applications to Everyday Life:

This phenomenon explains how air masses rise and sink due to warm temperatures and causing high and low pressure regions. As the air mass cools, it shrinks, allowing air from the surroundings to fill in above it, thus increasing the total mass of atmosphere above the surface, which then results in higher surface barometric pressures.

The pressure difference between the high pressure area and its lower-pressure surroundings causes a wind to develop flowing from higher to lower pressure. But because of the rotation of the Earth, the wind is deflected to the right (in the Northern Hemisphere) which then causes the wind to flow in a clockwise direction around the high pressure zone. All of this is caused by hot and cold air mixing, just like in our experiment. Convection currents arise in the atmosphere above warm land masses or seas, giving rise to sea breezes and land breezes, respectively

Another phenomenon that is described from this lab is natural convection. When a fluid is heated, it will naturally rise and allow the colder, denser fluid to drop. This is what happens in a homes hot water heater. This constant current allows incoming cold water that drops to the bottom of the heater to rise as its volume expands and density decreases, causing the cold water at the top to drop.

Photographs: Photographs of setup

Videos: